Accuracy and Sensitivity of Two Non–Invasive Tonometers for the Measurement of Intraocular Pressure (IOP) in the Mouse

Abstract

Abstract: : Purpose: To evaluate and compare the accuracy and the sensitivity of two prototype tonometers specifically developed for IOP measurement in the mouse. To determine what degree of pressure difference they can reliably detect. Methods:A duplicate prototype of the impact–rebound tonometer (I–R) described by Danias et al (IOVS 2003; 44: 1138 – 1141) and a prototype murine optical interference tonometer (OIT) utilizing a fiberoptic pressure sensor (Ahmed et al ARVO 2003; 44: E–Abstract 3336) were compared. After calibration on ex–vivo eyes, IOPs in two mouse strains, C57/BL6 (n=12) and DBA2J (n=34) were measured in vivo with the tonometers successively in alternating order. Three measurements were taken and averaged for each eye before cannulating the anterior chamber and invasively determining the "true" intraocular pressure. The error of the tonometric reading ΔIOP= IOP_tonometry – IOP_cannulation was plotted against the true intraocular pressure (IOP_tonometry). The 95% confidence intervals (CI) of the error were determined according to Bland–Altman. Results:Both tonometers yielded valid and almost equivalent calibration curves with individual R² = 0.99 (exponential) and 0.99 (linear) respectively. 1. All three methods detected similar mean percentile differences between the strains that were studied (22% for the I–R 16.5% for the OIT and 15% for cannulation respectively). 2. Both non–invasive tonometers introduced a systematic error in the IOP estimate detecting a small fraction of the IOP differential. The error appeared to be highly predictable and could be incorporated into a 2^nd order calibration formula (R² = 0.65 for both devices). 3. The 95%CIs of the error were acceptable for both devices measuring 2.36 mmHg for the I–R and 2.62 mmHg for the OIT respectively. Conclusions: Non–invasive tonometry in the mouse using impact–rebound and applanation methods is feasible. The instruments were able to reliably identify eyes that differed by less than 3 mmHg as different; a result that is comparable to the performance of Tonopen XL in the rat. Calibration efforts of noninvasive tonometers in mice (both in vivo and ex vivo) have the inherent difficulty that the eye is fixed in position. They cannot simulate the displacement of the eye within the orbit when the measurements are being obtained in vivo.

Keywords: apoptosis/cell death • intraocular pressure • ganglion cells